1. Field of the Invention
The invention relates in general to tunable microwave oscillators and in particular to oscillators for television receivers.
2. Description of the Prior Art
Tuners of television receivers of the prior art generally are constructed as a combination of mechanical and electronic change-over switches and separate VHF and UHF converter components for receiving the entire television frequency range of bands I, band III and bands IV/V. In order to be able to receive the entire frequency range from 54 to 890 MHz (in U.S.) which comprises more than 4 octaves normally two separately tunable oscillators have been used. The VHF oscillator for band III is switched to a different frequency and used in band I with the use of an additional inductor. It is also known to simplify the arrangement by using double conversion with a high first intermediate frequency rather than the single conversion. In order to cover the entire frequency range, the first oscillator of the tuner must be tunable in the range greater than 1 GHz and must be tunable over a range .DELTA. f of approximately 1 GHz.
The oscillators previously available for this frequency range have been very expensive and have been limited to frequency intervals of f.sub.max /f.sub.min of approximately 1.5. The conventional technique has used transistor oscillators with YIG resonators as the frequency determining components. However, these oscillators cannot be constructed with monolithic integration techniques. Also, the expense of the peripheral circuits and the expensive mechanical construction and the associated high construction costs do not lend such circuits to television receiver tuners.
If the principle of double conversion is used and a first IF of 1 GHz is used in a television tuner an electrical oscillator which is fully tunable over a frequency range from 1 to 2 GHz is required in order to convert useful frequencies from 0 to 1 GHz into the television IF position of 36 MHz. Therefore, it is desirable to have an oscillator having a frequency tuning interval of f.sub.max :f.sub.min greater than 2 as well as having adequate HF power.